Such high-voltage transformers are adequately known from the prior art. They are frequently parts of a test or measuring instrument, which may be mobile, in which the high voltage that can be tapped on at least one secondary winding is used as the test voltage for a component to be tested or for other measurement purposes.
In this process, an input a.c. voltage is applied to a primary winding of the transformer, which winding surrounds a ferromagnetic core of the transformer, which core is composed, for example, of iron or a laminated iron assembly. The magnetic field induced thereby in the transformer core then in turn induces, in the at least one secondary winding, a secondary voltage whose amplitude depends substantially on the ratio of the respective number of turns of the primary and secondary windings. In the high-voltage range of relevance here, particular care is advisable in connection with the electrical insulation of the transformer or of the secondary winding.
There exist high-voltage transformers of the type mentioned in the introduction with exactly one secondary winding, whose output a.c. voltage relative to ground signal is tapped. There are also known high-voltage transformers with in total two secondary windings, each delivering an output voltage phase-shifted by 180° relative to one another. In the case of secondary windings of identical configuration, it is therefore possible, by tapping the differential voltage, to achieve doubling of the maximum voltage than can be tapped on one secondary winding. Thus each of the secondary windings has to be designed only for a smaller output voltage, or for half of the output voltage in the case of identical constructions. However, it is expressly pointed out that the present invention is not limited to a particular transformer arrangement or geometry.
A coil support such as known from the prior art, supporting the secondary winding and made from electrically insulating material, insulates the secondary winding from the transformer core on the radially inward side relative to the coil geometry. For this purpose the coil support is advantageously provided with a central bore, through which, for example, one leg of the transformer core is passed. In the prior art, essentially two different insulation concepts are known for further electrical insulation of the secondary winding, which for its part frequently already comprises insulated wires.
In a first variant, known as dry transformers, the secondary winding is potted by means of a casting resin, which once cured provides adequate electrical insulation of the secondary winding toward the outside also. However, small defects (such as air inclusions or vacuum voids, if casting took place under vacuum conditions) in the dried casting resin may lead, as a result of voltage breakdown, to destruction, usually irreparable, of the transformer. In view of the high viscosity of the casting resins known in the prior art, great expense is needed to manufacture such dry transformers with the minimum possible reject percentage.
A further consideration is that it would be desirable to use thin winding wires for the secondary windings of novel high-voltage transformers, but this is not compatible with the high viscosity of the casting resin. Specifically, the casting resin is then no longer capable of closing, with adequate safety, any gaps that may be present between adjacent wires without forming defects. In the case of dry transformers, therefore, “preimpregnation” of the winding with a more fluid insulating medium is occasionally used even before the winding is potted in the resin. In this case also, however, the danger of undesired defects is very great, as is the manufacturing time and effort that must be expended to prevent such defective insulations. Dry transformers therefore rely mostly on relatively large wire diameters for the secondary winding.
Particularly in the use of high-voltage transformers of the type mentioned in the introduction in mobile test or measuring instruments, the total weight of the test or measuring instrument, determined substantially by the weight of the transformer, is an important criterion.
In the case of dry transformers of the aforesaid type, the casting resin already represents a not inconsiderable contribution to the total weight of the transformer, because of the layer thickness necessary for high-voltage insulation purposes and because of its density, which is usually in the range of approximately 1.3 to 1.7 g/cm3. The use of relatively thick wires for the secondary winding(s) also contributes disadvantageously to the weight of a dry transformer.
A second variant of high-voltage transformers uses, for insulation purposes, an insulating fluid, which surrounds the entire transformer within a transformer housing made of metal. In this case there is usually used an insulating oil, or else—frequently under pressure—an insulating gas (such as sulfur hexafluoride (SF6)).
In this case the entire transformer including transformer core and coil is surrounded by insulating oil. Because of the not inconsiderable thermal expansion of the insulating oil during operation of the transformer, or if the ambient temperature is high for other reasons, such oil-insulated transformers, if they are not used as stationary devices with continuous and cooled oil circulation, must provide a special expansion volume (for example, in the form of an expansion vessel), into which the insulating oil can expand as needed, as is the case, for example, in the oil transformer according to DE 1226119. The oil volume surrounding the entire transformer contributes substantially to the total weight of such a transformer, as does the weight of the metal housing.
Further oil transformers are known from CH 470738 and DE 714480, but they also include means for providing a separate expansion volume.